1. Field of the Invention
The present invention relates to an image signal processing device and an image signal processing method used to obtain luminance signals for individual pixels from a plurality of chrominance signals.
2. Description of the Prior Art
Conventional Method of Producing Luminance Signals
First, a conventionally typical method of producing luminance signals from the image signals output from a CCD (charge-coupled device) provided with a plurality of types of color filters will be described. Here, as shown at (a) in FIG. 8, the image signals are assumed to be output from a CCD provided with four types of color filters, namely color filters for M (magenta), G (green), Y (yellow), and C (cyan) colors. As shown at (a) in FIG. 8, the CCD has two types of columns of color filters arranged alternately, specifically columns in which color filters are arranged in the order of M, Y, G, and Y and columns in which color filters are arranged in the order of G, C, M, and C.
This CCD outputs image signals obtained from two adjacent rows in combination. Specifically, as shown at (b) in FIG. 8, for every two rows, the CCD outputs image signals M+Y, G+C, G+Y, and M+C, which are expressed also as C1=M+Y, C2=G+C, C3=G+Y, and C4=M+C, respectively. Where image signals are output in this way, the colors M, C, and Y are expressed, in terms of primary colors R (red), G (green), and B (blue), as M=R+B, C=G+B, and Y=R+G, respectively. Hence, the image signals C1, C2, C3, and C4 are expressed, in terms of primary colors R, G, and B, as C1=2R+G+B, C2=2G+B, C3=2G+R, and C4=2B+G+R, respectively.
Since the image signals output from the CCD have color components superimposed on luminance signals, those image signals have different signal levels in different columns in a colored portion of the image they represent, even if color saturation is low there, as shown at (c) in FIG. 8. The difference between C1 and C2 and the difference between C3 and C4 are color components. By passing such image signals through a low-pass filter, it is possible to obtain smoothed signals 2R+3G+2B, which are direct-current components, and these signals are used as luminance signals.
A conventional image signal processing device operating in this way is shown in FIG. 9. The image signal processing device shown in FIG. 9 is, for example, a device for extracting luminance signals from image signals output from an image-sensing device, such as a single-panel color CCD, provided with a plurality of types of filters. Suppose that, this image signal processing device is fed with image signals from the above-described CCD provided with four types of, i.e. M, G, Y, and C, color filters. As described above, the CCD outputs one image signal for every two pixels provided with two vertically adjacent filters.
When the image signal processing device is fed with image signals from the CCD, the image signals are fed to a line memory 51 and to a vertical-direction low-path filter (hereinafter referred to as the xe2x80x9cVLPFxe2x80x9d) 53. The image signals output from the line memory 51 are fed to a line memory 52 and to the VLPF 53, and the image signals output from the line memory 52 also are fed to the VLPF 53. In this way, image signals from one row after another are stored in the line memory 51 and then in the line memory 52, and thus image signals from three vertically adjacent rows are fed to the VLPF 53. Specifically, the image signals from the first row are fed from the line memory 52 to the VLPF 53, the image signals from the second row are fed from the line memory 51 to the VLPF 53, and the image signals from the third row are fed directly from the CCD to the VLPF 53.
The VLPF 53, fed with image signals in this way, produces image signals by taking averages of the first-row image signals fed thereto from the line memory 52 and the third-row image signals fed thereto directly from the CCD, and feeds the resulting image signals, together with the second-row image signals fed thereto from the line memory 51, to a horizontal-direction low-pass filter 54 (hereinafter referred to as the xe2x80x9cHLPFxe2x80x9d) 54. The HLPF 54 is fed with image signals from three columns, and produces image signals by taking averages of the image signals from the first and third columns. In this way, the VLPF 53 and the HLPF 54 calculate the signal levels of the image signals C1 to C4 for the individual pixels, and then the HLPF 54 feeds luminance signals of which the signal levels are equal to the averages of the signal levels of the thus calculated image signals C1 to C4 to a luminance signal processing circuit 55.
Specifically, for example, when the VLPF 53 and the HLPF 54 produce luminance signals for the pixels from which the image signals C1 are obtained, they calculate the signal levels of the image signals C2 to C4 plausibly, and then the HLPF 54 produces luminance signals of which the signal levels are equal to the averages of the signal levels of the image signals C2 to C4 thus plausibly calculated and the image signals C1. Thereafter, the luminance signal processing circuit 55 further varies the signal levels of the luminance signals fed thereto from the HLPF 54 by performing edge enhancement and other processing thereon, and then outputs the thus processed luminance signals.
In this way, luminance signals are produced from the image signals obtained from a solid-state image-sensing device such as a CCD. However, as shown in FIG. 8, the luminance signals thus produced are not luminance signals that correspond one to one to the individual image signals, but signals obtained by passing the image signals through low-pass filters, and are thus signals of which each has a signal level proportional to the average of two image signals. This makes it impossible to reproduce variations in luminance in the detail of the subject.
An object of the present invention is to provide an image signal processing device and an image signal processing method that produce luminance signals with which images can be reproduced with enhanced resolution and definition.
To achieve the above object, according to one aspect of the present invention, an image signal processing device for producing luminance signals from image signals output from a solid-state image-sensing device having a plurality of types of color filters arranged one for each of the pixels thereof is provided with: an image signal accumulator for accumulating, for each of the types of color filters provided for the pixels of the solid-state image-sensing device, the signal levels of the image signals output from those of the pixels which are sensing a low color saturation region in which color saturation is low; a transmitted light amount corrector for producing, based on correction constants given one for each of the types of color filters provided for the pixels of the solid-state image-sensing device, corrected image signals by correcting the image signals output from the pixels of the solid-state image-sensing device in order to counterbalance, for each of the types of color filters, the amounts of light transmitted through the color filters; a correction constant calculator for setting, based on the signal levels of the image signals accumulated for each of the types of color filters, the correction constants one for each of the types of color filters and feeding the thus set correction constants to the transmitted light amount corrector; a first luminance signal generator for smoothing the corrected image signal currently being fed thereto from the transmitted light amount corrector as obtained from the currently targeted pixel and the corrected image signals obtained from a plurality of pixels located in the neighborhood of the currently targeted pixel in order to produce a luminance signal for the currently targeted pixel; and a second luminance signal generator for newly producing a luminance signal for the currently targeted pixel based on the corrected image signal fed thereto from the transmitted light amount corrector as obtained from the currently targeted pixel and the luminance signal for the currently targeted pixel fed thereto from the first luminance signal generator.
According to another aspect of the present invention, an image signal processing method for producing luminance signals from image signals output from a solid-state image-sensing device having a plurality of types of color filters arranged one for each of the pixels thereof includes: a step of accumulating, for each of the types of color filters provided for the pixels of the solid-state image-sensing device, the signal levels of the image signals output from those of the pixels which are sensing a low color saturation region in which color saturation is low; a step of setting, based on the signal levels of the image signals accumulated for each of the types of color filters provided for the pixels of the solid-state image-sensing device, correction constants with which to counterbalance the amounts of light transmitted through the color filters provided for the pixels that are sensing the low color saturation region; a step of producing corrected image signals by multiplying by the correction constants the image signals output from the pixels of the solid-state image-sensing device that are sensing the low color saturation region; and a step of using the corrected image signals as luminance signals for the pixels of the solid-state image-sensing device that are sensing the low color saturation region.
According to still another aspect of the present invention, an image signal processing method for producing luminance signals from image signals output from a solid-state image-sensing device having a plurality of types of color filters arranged one for each of the pixels thereof includes: a step of accumulating, for each of the types of color filters provided for the pixels of the solid-state image-sensing device, the signal levels of the image signals output from those of the pixels which are sensing a low color saturation region in which color saturation is low; a step of setting, based on the signal levels of the image signals accumulated for each of the types of color filters provided for the pixels of the solid-state image-sensing device, correction constants with which to counterbalance the amounts of light transmitted through the color filters provided for the pixels that are sensing the low color saturation region; a step of producing corrected image signals by multiplying by the correction constants the image signals output from the pixels of the solid-state image-sensing device that are sensing the low color saturation region; and a step of using, as luminance signals for the image signals, signals produced by adding together the corrected image signals and smoothed image signals produced by smoothing the image signals obtained from one set after another of a plurality of adjacent pixels, wherein the corrected image signals and the smoothed image signals are added together with predetermined weights assigned thereto.